1. Field of the Invention
An object of the present invention is a method for measuring flows in a nuclear magnetic resonance (NMR) experiment. Experiments of this type have long been used for physical measurements. In recent years they have also been used in the medical field. As a matter of fact, NMR has lately proved to be one of the main techniques for investigating the inside of the human body. It does not cause any trauma and is not painful. While the earliest developments of this technique were related to knowledge of the architecture, anatomy and fixed parts of the body, there is now a change towards a demand for knowing the characteristics of moving parts of the body. For example, in the human body, there is a great need to know the distribution of blood flowing in the heart.
2. Description of the Prior Art
There are several known techniques for measuring flows in NMR experiments. In particular, the European patent application No. 84 307746.2, filed on 9th Nov. 1984, describes a method for reducing sensitivity to motion in images obtained by NMR. The flows, in this method, are depicted by means of two successive, complementary acquisitions. In a first acquisition, the effect of the motion of the moving, parts is not compensated for in the measurements: as a result, the data obtained gives only a picture of whatever is fixed in the body. In a second acquisition, the effects of the speed of the moving parts in the NMR signal are compensated for in such a way that these moving parts effectively contribute to the signal. By comparing the data acquired at the end of the second acquisition with the data acquired at the end of the first acquisition, data pertaining only to the moving parts is deduced. The drawback of this method is that it calls for two acquisitions. Moreover, for reasons which shall be explained further below, each acquisition should be by synchronizing the acquisitions on a phase of an examined cyclic movement in the body which is studied. In doing so, it is implicitly assumed that the moving phenomenon studied is stationary to a certain degree. This assumption that the phenomenon is periodically stationary is all the more unwarranted when it concerns prolonged periods. This is wholly the case when data is obtained at the end of two acquisitions which are already lengthy in themselves.
Another method has been presented by G. Bielke, S. Meindl, W. V. Seelen and P. Pfannenstiel of the Deutsche Klinik Fur Diagnostik, Wiesbaden, Federal Republic of Germany, at the SMRM's Fifth Congress on NMR medical imagery held at Montreal, Quebec, from 19th to 22nd Aug. 1987 (Volume 1 pp. 76 and 77). This technique is called the "Quantitative assessment of laminar volume flow by orthogonal excitation with multiple echoes". This method consists in exciting the magnetic moments of the nuclei of a body, in a given slice of this body. They are made to flip and, at each excitation sequence, the magnetic moments thus flipped, are subjected to a subsequent excitation, called a spin echo, which has the specific feature of having a large band. This means that the effect of this spin echo excitation is not restricted to the protons which remain in the slice (the fixed protons) but also concerns protons (those in motion) which have left the slice. To select the slice, a so-called selection encoding gradient is used. According to a known technique, a read gradient is used when detecting the signal. In the technique referred to, the read gradient has the specific feature of being oriented in the same direction as the selection gradient. The immediate result of this is that the moving parts then contribute to the NMR signal with, at the measuring instant, a frequency which is shifted with respect to the central resonance frequency. The shift represents the strength of the applied gradient (which is known) and the distance between the concerned protons in the slice in which they were at the excitation. In a way, the distance travelled by these protons is measured. For predetermined periods of experimenting, the speed of the protons considered is deduced from this distance. In practice, the image of the slice is projected in a plane perpendicular to itself: its projection resembles a line. The moving parts then appear in the image of the line as localized humps with regard to the alignment of this line. In a preferred way, the images are obtained with an imaging method known as the phase encoding method or also the 2DFT method. In this imaging technique, several measuring/excitation sequences are undertaken, and a second encoding gradient is applied between the excitation pulse and the spin echo pulse of each sequence. The value of this gradient changes from one sequence to another during the experiment. The image shown is an image obtained by projection in a plane containing the axis of this phase encoding gradient and the read axis (which, in the present case, is also the selection axis).
This latter technique has one essential drawback: the range of data presented is unfavorable to precisely that phenomenon which is to be shown. For the hump observed with respect to the alignment depends, as we have seen earlier, on the speed of the moving parts. For example, in medecin, this flexure may represent the blood in an artery or vein or in the heart. Now the speed diagram of a fluid is continuous. On the walls of the flow tube (i.e. the tube in which the flow occurs), the flow is almost nil whereas, at the center of the tube, it is at the maximum. The result of this is that the flexure on the line will look substantially like a portion of a parabola. The two wings of this parabola bear on the projected line: they represent those parts for which the flow is small. In an image of a slice of a real human body, it may be considered, for example, that the diameter of a flow tube of this type is about 1 cm, i.e. substantially 30 times smaller than the diameter of the body. Since the image is a projected image, at the place of the flexure the lines showing parts moving at low speeds can be seen intermingled with the lines showing fixed parts located so that they are vertical (because of the projection) to either side of the considered flow tube. Now the lines showing the fixed parts are substantially 30 times bigger (in fact 29 times=30-1) than the lines of the moving parts. In the end, these latter lines are not seen. The data on these moving parts appears like a noise with respect to the signal related to the fixed parts.
The invention removes the drawbacks referred to, by proposing a measurement of the flows of moving parts wherein a single acquisition is needed and wherein the image of the fixed parts is eliminated so as to increase the useful dynamic range of depiction of parts in motion. Essentially, in the invention, a predetermined slice of the body is excited by making the magnetic moments of the particles of the body placed in this slice flip, preferably by 90.degree., whether these particles are fixed particles or moving particles. During the sequence, by means of a subsequent excitation of the spin echo type, the scattering of the phase of the free precessional signal of the excited magnetic moments is caused to be reflected. This dispersal is due to non-homogeneity in the orienting field of the machine and, for this reason, this subsequent excitation is known as a reflecting excitation. In the invention, this reflecting excitation has the specific feature, unlike in the second prior art technique described, of having not an excessively large band but a normal one, and of being applied, in the presence of one and the same selection gradient, to a frequency which is shifted with respect to the central resonance frequency so that it causes a useful phase reflection only for particles which are now in a slice adjacent to the slice that was excited beforehand. The result of this is that, at the instant of reception, the revival of the NMR signal concerns only particles which have left the slice excited beforehand. The fixed particles, which have not left it, do not contribute to the NMR signal. The result of this is that the dynamic range of the received signal can be used for a better depiction of the diagram of flows. In the prior art method referred to, the reflecting pulse could not have large band but could be, in fact, a single frequency band applied without any selection gradient. This amounts to the same thing. Finally, in the invention, at the reading instant, a gradient field is also applied to the body, this gradient field being oriented in the same direction as the gradient used to make the selection. At each sequence of the experiment, the strength of a phase encoding gradient is also modified so as to have a series of measurements useful for the depiction of the image.